Spin-polarized current created by quantum point contacts with spin-orbit interaction

We propose a new idea to create spin-polarized currents using quantum point contact (QPC) with spin-orbit interaction.\footnote{M.\ Eto, T.\ Hayashi, and Y.\ Kurotani, J.\ Phys.\ Soc.\ Jpn.\ {\bf 74}, 1934 (2005).} Neither magnetic fields nor magnetic materials are required. By numerical studies using Green function recursion method,\footnote{T.\ Ando, Phys.\ Rev.\ B {\bf 44}, 8017 (1991).} we show that (i) the conductance is quantized in units of $2e^2/h$ even in the presence of spin-orbit (SO) interaction, (ii) the current is spin-polarized in the transverse direction, and (iii) a spin polarization of more than 50\% can be realized with SO interaction strength in InGaAs heterostructures. The spin polarization stems from the transition between subbands of different spins during the transport through QPC. The spin-polarization ratio is determined by the adiabaticity of the transition, which is evaluated by the Landau-Zener theory. Since the two-terminal devices with QPC are easy to fabricate on semiconductors, they may be utilized for the spin injection in the spintronics.